Add support for running the regalloc2 checker. (#4043)

With these fixes, all this PR has to do is instantiate and run the
checker on the `regalloc2::Output`. This is off by default, and is
enabled by setting the `regalloc_checker` Cranelift option.

This restores the old functionality provided by e.g. the
`backtracking_checked` regalloc algorithm setting rather than
`backtracking` when we were still on regalloc.rs.

crates/wasmtime/src/engine.rs

@@ -336,6 +336,7 @@ impl Engine {
             | "enable_float"
             | "enable_simd"
             | "enable_verifier"
+            | "regalloc_checker"
             | "is_pic"
             | "machine_code_cfg_info"
             | "tls_model" // wasmtime doesn't use tls right now